Pickoff mechanisms have been in use for decades in automated letter sorting machines such as MLOCR and DBCS machines used by the U.S. Postal Service and private presort bureaus, as described, for example, in U.S. Pat. Nos. 5,109,987 (Daboub) and 6,679,491 (Luebben et al). The feeder section of the machine includes an unloading table where mail for sorting is manually placed edgewise to form a stack. The stack is advanced incrementally towards the pickoff mechanism which functions to feed mail pieces one at a time into a pinch belt conveyor system for sorting.
Known pickoff mechanisms comprise a series of rubber belts wound over a drive roller and a follower roller. The belts engage the endmost mail piece of the stack and rely on friction to pull it sideways off of the stack and into the entry nip of the pinch belt conveyor. Friction is created by the pressure of the mail stack as it advances into contact with the pickoff belts. The stack is carried by a horizontal belt conveyor, and its remote end is supported by a paddle movably mounted on a frame of the feeder. The paddle and belt are synchronized to move the stack forward in increments. This is controlled by a letter present sensor, for example, a mechanical proximity switch using a spring arm which indicates to the feeder controller that the end of the stack is in engagement with the outer face of the pickoff belts.
Some known pickoff designs rely on keeping the stack under pressure against the pickoff belts to create sufficient friction so that the pickoff operation proceeds smoothly at high speed. In practice, mail pieces are not uniform and sometimes slip against the pickoff belts, delaying feeding of the mail piece to the pinch belts. To remedy this, vacuum-assisted pickoff mechanisms were devised wherein suction is applied to the endmost mail piece through holes in the belts. This prevents slipping of mail pieces to a greater extent, but not entirely. The problem becomes more difficult when the incoming mail in the stack includes mail pieces of different sizes and thicknesses, such that some require more frictional force to feed than others. Present pickoff mechanisms have no means of adjusting to compensate for variations in mail piece characteristics. The present invention seeks to remedy this limitation, and in so doing improved performance of the conveyor as a whole by improving throughput.